


Souls Sin

by Fancyfandomlife



Series: Therapist Gaster [2]
Category: Undertale (Video Game)
Genre: AFAB Non Binary Reader, Biting, Consensual Sex, Exploration, Foreplay, Interspecies Relationship(s), Interspecies Sex, No Plot/Plotless, Other, Other Additional Tags to Be Added, Shameless Smut, Smut
Language: English
Status: In-Progress
Published: 2018-11-29
Updated: 2018-11-29
Packaged: 2019-09-02 03:44:30
Rating: Mature
Warnings: Creator Chose Not To Use Archive Warnings
Chapters: 2
Words: 3,978
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/16778944
Author URL: https://archiveofourown.org/users/Fancyfandomlife/pseuds/Fancyfandomlife
Summary: Any Risque or outright Smut I have written involving my Therapist!Gaster and Reader character from Souls Therapy. I highly recommend you read that before this.Chapter Ratings:Lime: Delicious and sweet, usually just foreplay or really heavy petting I don't want in the main story tldr, pro'ly just shirtlessness and ass grabbing.Lemon: I mean, it's foreplay and some other stuff, but I bet you couldn't get a clear image in your brain.Smut: I Am Describing Everything and the likelihood it was written at 3am with my last two brain cells is 200%





	1. Souls Therapy Chapter 5.2 LIME

**Author's Note:**

> I have had some things written for a while, and I need the story to progress before I can post some of them, BUT, All You Need To Know is I have zero chill and what you will find in future chapters could just Not Be your cup of tea.
> 
> I Can Promise you he never gets called daddy though, ew.
> 
> To start this off, let's do something familiar, shall we? It's 12am on a Thursday when I post this so I can say TBT to Chapter 5.2!

“Yes we shall.” you chuckle and he pulls your hips forward till you are flush against his torso. You lean into him and place your lips on his maw, they open slightly and his tongue slips through, softly lapping at your lips. Bound and determined to peeve him, you begrudgingly keep your lips pursed and he growls in irritation.  
“My sweet little (Y/N)…” his tongue coils back into his mouth, “I would be rather pleased if you cooperated. Or else…” his right hand trails up your back to your neck, his left trailing from your thigh to your hip, “face _repercussions._ ” his hands flex, squeezing your hip and nape of your neck.  
“Mmm ’Repercussion’ has never sounded so sexy and tempting.” you giggle, wrapping your arms around his neck and the back of the chair. He simply smirks and wraps his hand tighter around the small of your back, forcing you to arch backwards in the small space provided. He chuckles, dipping his head into the crook of your neck.  
“You smell phenomenal.” he tugs at your jacket and shirt collar  
“How do you smell? You don’t even have a nose.”  
“My tongue dear. Much like a snake would.” you feel the glowing appendage touch your skin, a shiver runs down your spine.  
“Hmm…” you throw your head back as his tongue moves up to your jaw, your hips involuntarily buck forwards, “That tickles a bit.” he growls, no you correct that statement, _purrs_ in satisfaction while he massages your vertebrae, continuing to ravish your neck.

“Oh, my dear~ Such lovely lewd noises you make.” he leans away from your neck, his right hand trails around your neck, under your ear, across your jaw line, stopping at your chin, “What lovely sound happens when I… **_do this?_** ” he dips back to the right side of your neck and bites down with steady pressure on your sternomastoid. Enough to pull a wince and whine from you, but instantly you start blushing with realization… _‘I like this’, “_ ** _Oh my, my._** What a delightful reaction. I felt your Blood Pressure rise when I did that. So exciting~” he laps at the spot he had just bitten, pulling your chin up rather than holding it in place, “Would you like another love bite? Or shall I leave your poor, bruising jugular alone?” you tentatively lick your lips.  
“M-maybe just one m-more. To make it even?”  
“Mm, as you wish.” his hand jerks your face forward and you feel an immediate pin-prick on your left side, this one closer to your hairline, easier to conceal. The pain from this bite though, was much more severe than the last, causing you to yelp slightly in pain, letting out a long moan afterwards. You feel his jaw loosen and him pull away, letting go of your face and letting his arm go slack, “You should leave. Immediately. Before you regret something.”  
“G-Gaster?” you retract your arms from around the seat and his neck.

“P-please. I… I **highly** suggest that you leave. Immediately.” one of his gloved hands covers his mouth and cheeks, “It is nothing against you. I am giving you an out. Take it before I choose to not control myself.” he unlocks the doors and opens his.  
“Um… okay?” I’ll ah… contact you later?” you ask, grabbing your stuff from the abandoned passenger side seat. The only response was a silent nod as you slipped out from between the Steering wheel and Gaster. Walking safely around the vehicle to the sidewalk you watch as Gaster drives away, unbeknownst to either of you, was a smiling figure in the window of the truck parked across the street.


	2. Body Exploration 1: Hotel LIME

**Summary for the Chapter:**

> You and Gaster get back to your hotel from Coffee With Alphys (Chapter 17 ST), and you get a little curious.

**Notes for the Chapter:**

> From personal experience: you /really/ don't want to rush into doing the do right away. You want to talk to your partner and learn their likes and dislikes, figure out pet names, 'safe' words/actions. This kind of stuff is super important, makes the first time (wink wink) so much more enjoyable.

When the two of you get back to the hotel and up to your room you turn to Gaster.  
“Hey, I have a super personal question and I have no way of sugar coating it.” You say, he looks at you sceptically.  
“I am an open book but I have no idea what question would cause you to be so blunt.” he says, taking his coat off.  
“Alphys said your previous partners never shut up about you, but you said it was about their personalities.” he freezes mid-motion.  
“Yes, go on?”  
“And then she mentioned you’re _experienced_.” you fidget.  
“Do go on.”  
“I mean, this is so awkward to ask, but you've had experience with humans before?” he smiles, then continues to hang his jacket and turns to you.  
“Yes, I have. What is your question, little one?” you stand awkwardly.  
“Umm I was just, um, curious about what you know?” you ask, not even sure if that's what you wanted to ask.  
“Depends. Are you comfortable with a physical demonstration or would you prefer bullet points?” that first one was an invitation for something delicious and naughty.  
“Er, in between? I don't even know if that's possible?” you're not even sure what you're doing right now.  
“I suggest, for the best results, that you do not bind. You may damage your ribs.” he suggests, “I will be waiting in the chair.” he teleports past you, then sits in the chair, crossing his legs and leaning on the desk, arm propping his head up. You rush into the bathroom, taking your shirt off. Then you do the binder shimy and take it off as well. Throwing your shirt back on you walk out, Gaster hasn't moved. He gives a ‘come hither’ motion with his left hand and you walk up to him, “If you are hesitant, we can still use bullet points.” he assures you.  
“No, I'm good.” his hands reach out to stroke your arms.  
“You are certain?” he confirms.  
“Yes.”

“If at any time you are done, let me know vocally, or if you are unable to-” what does he mean by that?! “Pull my collar twice.” he demonstrates, “Does that work for you?” he asks, you nod silently. He leans forward a hand behind your head and he starts kissing you. This is long, sweet and passionate. His tongue is lazy compared to normal, but somehow that's okay. You lazily grip his turtleneck, holding his face close as his tongue drags out of your mouth. “I wish to try something, a trick of mine that some have found pleasurable, you may feel unable to breathe, but do not fret. Breathe through your nose and watch your exhales.” he dips back in to kiss you, you welcome his slightly more hungry tongue, but it immediately shoots to the back of your throat and you freeze up, whimpering slightly. He holds it still, ‘breath through my nose’ you remind yourself, taking a small breath, you feel his tongue start to move and moan against it. You can feel it wiggling around, before it pulls out slightly, “Short breaths.” he reminds you, you take a breath, and his tongue plunges again. This time you close your eyes and grip his sweater tighter, still whimpering under his touch. He drags his hand down your back, pulling you closer to him, he pulls his tongue out completely, “Thoughts?” he asks, you open your eyes slowly, he’s pulling away.

“Holy shit.” you exhale, trying to catch your breath, “I’m weak.” you put extra weight on to his shoulders and chest as you lean on him. He chuckles.  
“There is more if you would like to experience it.”  
“Hm, a little more won’t hurt.” you mumble.  
“As you wish.” he grabs your hips and lifts you effortlessly onto the desk as he stands. It all feels very fluent to you. He leans in like he’s going to kiss you, but dips to your jawline, you place your hands behind you to support yourself as he pushes your chin up, “Inform me if I am going too hard.” he whispers by your ear, then licks it, trailing his tongue down your jaw and onto your neck.

“This isn’t the first time you’ve bit my ne-Oh!~” his teeth graze against your neck, further back than usual, and you feel his maw open, then he takes half of your neck into his mouth and holds you there. Gentle pressure is being applied constantly. You move your legs around his body, his dominant hand grips your thigh, moving up and down, as his non-dominant holds him up on the desk. You take in a breath through your nose, holding it, and then letting it out halfway as his jaw tightens, you suck in another breath, “O-okay Gaster! Tha-that’s tight enough!” You moan, his jaw stops tightening, and you feel his tongue run up and down your neck, his left-hand runs up your thigh past your hip, and slowly starts pulling the side of your t-shirt up. You get scared and your body tenses, Gaster’s hand stops just below your rib. You fear moving, his teeth at your neck, predatory and sharp, his hand on your body, holding you there, and his body looming over you, caging you in. His teeth let go and you let out a sigh you didn’t know you’ve been holding in.

“Are you alright my dear?” he asks, concern on his face as he pulls his hand away, letting the t-shirt remain ridden up on your body. Your legs are still wrapped around him, locked at the ankles and heels resting on his backbone.  
“Y-yeah.” your voice is shaky, “I just, didn’t know what to make of it.” a shiver goes down your spine.  
“Was there a part that made you uncomfortable?” he asks, left hand supporting his body, the right rubs your neck where he had bitten, his eyes are focused there.  
“N-no, not really? I mean, I freaked out when you pulled my shirt up but I was fine once it stopped.”  
“Dysphoria, perhaps?” he asks.  
“N-no. I don’t really experience it that much at all.” your words sit in the air for a moment, tense and uncertain, “My neck hurts.” he reach up to your neck where he’s rubbing.  
“You may wish to wear a concealer on your neck. Do you have a scarf?” your eyes meet, you open yours wide.  
“Is it that bad?” you move off the desk and walk to the bathroom, looking in the mirror, you look in horror, yet a chill runs down your spine, “That’s a whole ass _bite_. Not like a hickey or what you normally do!?” You turn to see him standing in the doorway, leaning against it.  
“A claim mark. It is not permanent and will be gone in a few days. “  
“I don’t have any make-up with me! OR a scarf!” your voice is becoming hysterical.  
“Relax dear. People at the lab will not ask about your neck.” he walks in, coming up behind you, bending down to your height, a hand on your opposite shoulder, “Are you self conscious, little one?” his other hand moves your hand off, holding it back as he makes eye contact while licking the mark.  
“D-define _self_ conscious?” you squeak, the hand on your shoulder snakes down your arm around to your stomach.  
“Perhaps ‘wary’ would have been more accurate.” he says before licking the shell of your ear.  
“Mh, probably.” he hasn’t taken his eyes from yours, you watch in the mirror as his tongue works on your neck, “I’ve just never… had a clean mark like this. “  
“Human teeth are blunt, the marks fade together easy. I have only previously given you human ‘love bites’, only enough pressure and suction to cause irritation to the skin.” he lets go of the hand that had previously been occupying your neck before his mouth had, “This is _my_ love bite.” he strokes it tenderly, his face softening,

“Okay so if this is your softcore demonstration of what you can do, what are the bullet points?” you ask, leaning into him. His hand pets your hair.  
“I have experience with many types of people.”  
“Alphy already mentioned you've brought ‘misters' back before, sorry don't be mad at her.” you say quickly.  
“I have. I show no prejudice or preference for what someone, to use lax terms, ‘has in their pants’.” the hand on your stomach trails up, “As you know, I am an adequate kisser.”  
“Hmm. That's putting it lightly.” you tease, body shivering. There's now a summoned hand holding your hand away from your body.  
“I have been informed by partners I am exceptional at oral.”  
“Even people with-?” you let the question hang, not wanting to really ruin the moment with vulgar language.  
“Yes.” he replies shortly, standing. He purrs, “I just wish to touch your body all over. Stimulate your tactile senses.” you throw your head back, moaning lightly as both his real hands touch your head.  
“We’re a ways off from that yet, Gast.” you can’t help but push against his hands, wanting to feel them more.  
“Perhaps one day we may get there?” he asks.  
“Mhm. Yeah… maybe.” his hands leave your hair and he steps away, afraid you said something wrong you open your eyes to look in the mirror.  
“I do not mean to force any expectations on you.” he clarifies, “I am just trying to ascertain your boundaries and comfort zones.”  
“Most people just go exploring and stop - or don’t - when they try to get in bed with someone.” you turn to face him, placing your hands flat against him, just under his chest, trailing down and to the sides, “They feel and explore, find out what makes their partner tick.” you grab his sweater and pull him against you, “And I find that to be a fun activity with someone you trust, wouldn’t you agree?” your neck is hurting from craning it up at him, but to see his eyes flicker in acknowledgement for the words you spoke.  
“Then perhaps I shall try again?” he purrs.  
“Just don’t hold me down like a wolf with its teeth sunk into a rabbit. Less predator and prey and more…” you struggle to find a comparison, “... something other than that.” he scoffs.  
“At a loss for words, little one?”  
“Mm, no. Just don’t keep me in a compromising position, please-” you smile leaning your chin on his chest,”-sir?” he growls, and you feel a sudden cold but then the warmth again, he is leaning over you on the bed.  
“I warned you to be mindful of your use of ‘sir’.” he purrs down at you.  
“And it still got me exactly what I wanted. I don’t see a problem with it.” you snake your hands around his neck, pulling him closer.  
“Neither do I, little one.” he coos, allowing his left hand to roam up your side.  
“Can you at least take your gloves off first?” you ask, batting your eyelashes at him. He starts pulling away, and you let your arms loose, but sit up with him. He takes the gloves off one by one and tosses them on the bedside table. You scoot further back on the bed as he takes his gloves off, and when he finishes he crawls on the bed above you.  
“Where were we?” he purrs, you move your leg to the outside of his body, and place the same hand on his shoulder, allowing the other to glide along the arm on the side opposite.  
“I think we were getting a little more…” you push up with your leg, pushing his shoulder and pulling his arm, flipping over so you’re sitting on top, “... comfortable~” he let out a whine as you pinned him down, you lean into his face, “May I explore, sir?” you ask coyly, already allowing the hands on his shoulders to run across them lightly.  
“As you wish.” he puts his hands behind his head, a summoned hand grabs one of the hotel pillows and places it under his head. He’s grinning at you, you can see his tongue flicking back and forth against the back of his teeth.

You sit up, and place all your weight right on top of his crotch area, using your hands to run under his turtleneck. You feel some muscle on his stomach, but can’t comfortably get higher, so you drag them back down, letting your nails graze slightly, he lets out a long purr.

“Please take your shirt off.” you instruct, and he wastes no time summoning two hands to support your upper back before pushing himself up, and sitting straight. He raises his hands to pull it off from the back of the neck, which causes you to lean back, but luckily his summoned hands are there to support you. He takes the whole shirt off and hands it to summoned hands, they start folding it, but your focus is so directly centred on his broad, slightly toned chest and the scar running over his right shoulder. He lays down again, placing his hands behind his head. He purrs for you to continue, and you lean forward to touch the scar. It is the only mark on his otherwise blank canvas of a body. You trail a finger from the thin end of it and move it along the edge, it gets thicker as it nears the shoulder. Gaster is watching you the whole time, silently observing.

“Most have just asked what it is from.” he finally comments after you've been slowly trailing the mark. You shake your head.  
“No, that'll just ruin the moment.” you muse, “I don't mind not knowing. It's just…” you sit up, “You're white as a canvas and yet there's only one brush stroke, where I thought there might be more.” you smile at him, “I feel less conscious about myself now.”  
“Well that is a good thing, is it not?” you start pushing your hands up his body, running over his lean abs.  
“Yeah, it's a positive thing. “ You stop just under his ribs again, “So odd still how you don't breathe.” his hands come out from behind his head and reach down to grab the back of your thighs.  
“Do not pay it much mind. It is an anatomical aspect of my body, much like our differences in height.” the hands pull you forward, you yelp slightly as your hands slide further up, “Which means we must compensate for the difference.” You’re sitting just above his belt now, “Do I have the honour of being your equal, or will you remain clothed?” he teases, sliding his hands from your thighs up your back, “Or must I lay at your mercy.”  
“In a bit.” you run your hands on his body more, then lean down, “Are you sensitive on your neck?”  
“Very. It is why I wear collars.” he explains.  
“Interesting.” you dip down, kissing his neck, he shivers under you, you kiss again, trailing them down to his pronounced collarbone, “So if I wanted to bug you…” you look up at him, “I’d need to get you _hot under the collar_?” he laughs at you.  
“You understand that means angry, my dear.”

“Yeah.” you dip back down and nip his neck, “Making you agitated is fun too. You usually give me rough kisses or something when I manage that.” you nip him again, he whines, “You must really like this if you’re whining at me.” his hands trail up your back, pulling the shirt with him, his claws trail your spine and lightly scratch your skin. You shudder at the contact, sucking in a shaky breath.  
“I do so much enjoy your company.” he mumbles, hands stopping under your shoulder blades, “May I?” he asks, and a shiver runs down your spine. You sit up, and Gaster pulls the shirt up more, exposing your chest. You help him take it off, then toss it off the bed. You are now sitting on him with nothing but a pair of jeans on, staring down at him, as he growls low, “How lovely you are, dearest.” he trails his hands up your sides, “Simply phenomenal.”  
“Okay, smooth talker. You’ve said your words of honey, now work your magic touch.” He smirks, the left side of his face raises, almost meeting the bottom of the crack. He sits up, and you roll over, bringing his body with you. You wrap your legs around him again, letting your hands rest above your head. He dips in, licking the love bite he gave you, then trails nips down your body, much like you did on him. Biting down harder on your collarbone, you gasp, his hand trails over your nipple in a feather-light brush, turning your gasp into a moan, he repeats the action, moving his head up to the opposite side while gently teasing your sensitive nub, “You have two hands-” you stop, correcting yourself, “You have _several_ hands Gaster. Can’t you multitask?” you tease, he growls, using his free hand to push your hip down.

His tongue drags down to your other nipple, the odd texture and surprising lack of moisture providing plenty of friction, and your hip arches against his hand, a whine escapes you.  
“O-okay, fuck- maybe you can multitask.” his teeth just mildly skim against you, a breathy moan escapes from you, “Lord, okay, you can multitask. Holy shit Gaster.” His mouth and hand switch, he remains diligently focussed on his task. Your breath is coming quicker and in shorter bursts, “H-hey Gast.” you grab his head, he looks up, “Mind doing the tongue thing again, with the kiss and- mph!” he lifts your head up and shoves his tongue in immediately, your hands on his head fell to his neck, and they tense as he starts sitting up, his tongue massaging yours, eyes closed, you moan. A hand supports your back, while the other holds his weight on the bed. With a slight jerking motion, he pulls you closer and shoves straight past your tongue, causing you to gag again, squinting your eyes trying to calm your muscles. The hand on your back pulls your hair gently, making you tilt your head… opening your throat for Gaster. You try moaning, but it’s cut short as you start feeling light-headed, and double tap Gaster’s shoulder lazily. He retracts and you gasp, he pulls his face away from yours, and you open your eyes slowly. Your lips are soaked in saliva, and a small string still connects to Gaster's tongue. He dips to your ‘love bite’, licking the rest of the saliva off his tongue, before allowing it to retreat back into his maw.

“Was that satisfactory, my sweet little Y/N?” he purrs.  
“Mh.” you groan, clinging to him tighter as you catch your breath. He smiles, his teeth slightly parted.  
“I am thrilled you enjoyed yourself.” he rubs against the side of your head, then sets you down gently. You unwrap your legs from around his torso, and he gets off the bed. He looks down at you, and you start becoming self-conscious, so you cross your arms over your chest, “How adorable. You are shy.” he walks away, then bends down to pick up your shirt off the floor. He turns and tosses it to you, it lands by your hip, “If you wish to wear it you may.” you sit up, grabbing the shirt and putting it back on. He doesn’t put his on, coming back to arrange the pillows again.  
“You just gonna parade around like that?” you ask, he looks back at you, then chuckles.  
“No. I am going to lay down and relax, you may join me if you wish.” he sits on the edge of the bed by the pillows, which he propped up, then lifts his legs up and leans against the pillow pile. You crawl over after getting your shirt on and place a hand on his right shoulder, then kiss his cheek.  
“You’re such a nerd.”  
“Thank You, I do try.” he teases, lifting his hand to stroke your back.

**Notes for the Chapter:**

> Any questions regarding something in this chapter can be sent to my [Personal Tumblr](http://myfancyfandomlife.tumblr.com), or left in the comments below.
> 
> Also feel free to tease Gaster [here](http://souls-therapy-gaster.tumblr.com), he only bites a little ;)


End file.
